ShiftAndResample¶

class baseband.tasks.ShiftAndResample(ih, shift, offset=None, whence='start', *, lo=None, pad=64, samples_per_frame=None)[source] [edit on github]¶

Bases: baseband_tasks.convolution.Convolve

Shift and optionally resample a stream in time.

The shift is added to the sample times, and the stream is optionally resampled to ensure a sample falls on the given offset.

If the shift corresponds to a time delay, and the signal was recorded after mixing, the phases should be adjusted, which can be done by passing in the local oscillator frequency. For an upper sideband, the phase correction is,

\[\phi = - \tau f_{lo}.\]

For the lower sideband, the rotation is in the opposite direction.

Parameters

ihtask or baseband stream reader: Input data stream, with time as the first axis.
shiftfloat, float array-like, or Quantity: Amount by which to shift samples in time, as (float) samples, or a quantity with units of time. Should broadcast to the sample shape.
offsetfloat, Quantity, or Time: Offset that the output stream should include. Can be an absolute time, or a (float) number of samples or time offset relative to the start of the underlying stream. The default of None implies that the output stream is free to adjust. Hence, if a single shift is given, all that will happen is a change in start_time. To ensure the grid stays fixed, pass in 0.
whence{0, 1, 2, ‘start’, ‘current’, or ‘end’}, optional: Like regular seek, the offset is taken to be from the start if whence=0 (default), from the current position if 1, and from the end if 2. One can alternativey use ‘start’, ‘current’, or ‘end’ for 0, 1, or 2, respectively. Ignored if offset is a time.
loQuantity, or None, optional: Local oscillator frequency. Should be passed in when the signal was mixed down before sampling and the shift should be interpreted as a time delay (rather than a clock correction). For raw data, this can usually just be if.frequency. But for channelized data, the actual frequency needs to be passed in. If data were recorded without mixing (like for CHIME), no phase correction is necessary and one should pass in None. Default: None.
padint, optional: Padding to apply on each side when shifting data. This sets the size of the sinc function which which the data is convolved (see below). Numerical tests suggest that with the default of 64, the accuracy is better than 0.1%.
samples_per_frameint, optional: Number of resampled samples which should be produced in one go. The number of input samples used will be larger by 2*pad+1. If not given, works on the larger of the samples per frame from If not given, the larger of the sampler per frame in the underlying stream or 14 times the padding (to ensure ~87.5% efficiency).

See also

Resample: resample a stream to a new grid, without time shifts
ShiftSamples: shift channels by integer number of samples
TimeDelay: delay a complex stream, changing phases but no resampling
baseband_tasks.base.SetAttribute: change start time without resampling

Notes

The precision of the shifting and resampling is controlled by pad, as it sets the length of the response, which consists of a sinc function combined with a Hann window,

\[\begin{split}R(x) &= {\rm sinc}(x-s) \cos^2(\pi(x-s)/L),\\ -{\rm pad} &\le x \le {\rm pad},\quad L = 2{\rm pad} + 2\end{split}\]

Here, \(s\) is the fractional pixel offset. Note that \(L\) is chosen such that \(R(\pm{\rm pad})\) is still non-zero.

The convolution is done in the Fourier domain, and the Hann window, combined with with the padding done when reading, ensures there is no aliasing between the front and the back of each frame. There is, however, aliasing near the Nyquist frequency. For most recorded signals, this will not be important, as the band-pass filter will likely have ensured there is very little signal near the band edges. For channelized data, however, it may be more problematic and some pre-filtering stage may be necessary.

Attributes Summary

`closed`
`complex_data`
`dtype`	Data type of the output.
`meta`
`ndim`	Number of dimensions of the output.
`offset`
`sample_rate`	Number of complete samples per second.
`sample_shape`	Shape of a complete sample.
`samples_per_frame`	Number of samples per frame of data.
`shape`	Shape of the output.
`size`	Number of component samples in the output.
`start_time`	Start time of the output.
`stop_time`	Time at the end of the output, just after the last sample.
`time`	Time of the sample pointer's current offset in the output.

Methods Summary

`close`()	Close task.
`read`([count, out])	Read a number of complete samples.
`seek`(offset[, whence])	Change the sample pointer position.
`task`(data)
`tell`([unit])	Current offset in the file.

Attributes Documentation

closed = False¶

complex_data¶

dtype¶: Data type of the output.

meta¶

ndim¶: Number of dimensions of the output.

offset = 0¶

sample_rate¶: Number of complete samples per second.

sample_shape¶: Shape of a complete sample.

samples_per_frame¶

Number of samples per frame of data.

For compatibility with file readers, to help indicate what a nominal chunk of data is.

shape¶: Shape of the output.

size¶: Number of component samples in the output.

start_time¶

Start time of the output.

Navigation

ShiftAndResample¶